This invention relates to an improved cathode mixture comprising manganese dioxide and carbon fibers, particularly graphitized mesophase pitch-based carbon fibers.
Conventional alkaline electrochemical cells are formed of a cylindrical casing. The casing is initially formed with an enlarged open end and opposing closed end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing at the closed end forms the positive terminal.
Primary alkaline electrochemical cells typically include a zinc anode active material, an alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose or cellulosic and polyvinylalcohol fibers. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector, which is electrically connected to the negative terminal end cap. The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. The cathode typically includes particulate manganese dioxide as the electrochemically active material admixed with an electrically conductive additive, typically graphite material, to enhance electrical conductivity. Optionally, polymeric binders, and other additives, such as titanium-containing compounds can be added to the cathode.
Since manganese dioxide typically exhibits relatively low electric conductivity, an electrically conductive additive is needed to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite.
It is desirable for a primary alkaline battery to have a high discharge capacity (i.e., long service life). Since commercial cell sizes have been fixed, it is known that the useful service life of a cell can be enhanced by packing greater amounts of the electrode active materials into the cell. However, such approach has practical limitations such as, for example, if the electrode active material is packed too densely in the cell, the rates of electrochemical reactions during cell discharge can be reduced, in turn reducing service life. Other deleterious effects such as cell polarization can occur as well. Polarization limits the mobility of ions within both the electrolyte and the electrodes, which in turn degrades cell performance and service life. Although the amount of active material included in the cathode typically can be increased by decreasing the amount of non-electrochemically active materials such as polymeric binder or conductive additive, a sufficient quantity of conductive additive must be maintained to ensure an adequate level of bulk conductivity in the cathode. Thus, the total active cathode material is effectively limited by the amount of conductive additive required to provide an adequate level of conductivity.
Further, it is highly desirable to enhance the performance of an alkaline cell at high rates of discharge. Typically, this is accomplished by increasing the fraction of conductive additive in the cathode in order to increase overall (bulk) electric conductivity of the cathode. The fraction of conductive additive within the cathode must be sufficient to form a suitable network of conductive particles. Typically, when the conductive additive is a conductive carbon, about 3 to 15, desirably between 4 to 10 weight percent of the total mixture are required. However, an increase in the amount of conductive carbon produces a corresponding decrease in the amount of active cathode material, giving lower service life. Conventional powdery conductive carbons such as acetylene black have large volume in a certain weight but lower electric conductivity than the flaky, crystalline natural or synthetic graphite, which possess a three-dimensional crystal structure as described below. The less conductive nature of carbon blacks leads to high carbon content in cathode electrode and less electrochemically active material, in turn, to the shorter service life of electrochemical cells.
To increase the electric conductivity of carbonaceous materials, a thermal process known as graphitization is applied to convert carbons into graphite material or graphite product. Such a graphitic product is characterized by a distinctive three-dimensional graphitic crystal structure. The crystalline structure is composed of individual unit cells which are repeatable in the xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d directions. The graphite crystalline structure has a unit cell which is generally of a three dimensional hexagonal (six sided) shape. The base of the hexaganol unit is defined by a hexaganol plane of sides xe2x80x9caxe2x80x9d of equal size and separated from each other by 120 degrees. The hexaganol plane defines the xe2x80x9caxe2x80x9d direction of the crystalline structure. The thickness of the unit cell is defined by the height of the unit cell defined by the axis xe2x80x9ccxe2x80x9d which is perpendicular to said hexagonal plane lying in the xe2x80x9caxe2x80x9d direction. A reference to such hexaganol unit cells using the same standard nomenclature can be found, for example, in F. Daniels and R. Alberty, Physical Chemistry, 2nd Edition, John Wiley and Sons (1961), pp. 622-623. The unit cells are repeatable in the xe2x80x9caxe2x80x9d direction and xe2x80x9ccxe2x80x9d direction up to a point where they abruptly change orientation. This defines the bounds of the crystalline structure in the xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d directions. Different graphites have different number of repeatable unit cell in the xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d direction. (The size of each repeatable unit cell for graphites will be the same.) The size of the crystalline structure (crystalline size) for a specific graphite can be defined by the distance La of the crystalline structure in the xe2x80x9caxe2x80x9d direction which spans the total number of repeat units in the xe2x80x9caxe2x80x9d direction, and the distance Lc in the xe2x80x9ccxe2x80x9d direction which spans the total number of repeat units in the xe2x80x9ccxe2x80x9d direction. The number of unit cells in the xe2x80x9caxe2x80x9d direction can be determined by dividing the distance La by the size of the unit cell in the xe2x80x9caxe2x80x9d direction. Conversely, the number of unit cells in the xe2x80x9ccxe2x80x9d direction can be determined by dividing the distance Lc by the size of the unit cell in the xe2x80x9ccxe2x80x9d direction.
In most graphitic products, for example, natural graphite, the Lc and La distances defining the crystalline size as measured by x-ray diffraction are typically in the range of 1000 to 3000 Angstrom. Expanded graphite, a typical exfoliated graphite, can have a large La of about 500 to 1000 but typically a smaller Lc of about 300 to 1000, typically between about 400 to 600 Angstrom due to chemical exfoliation in this direction. Due to the anisotropy of graphite in the xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d directions of the crystal unit cell, the La normally contributes to electric and thermal conductivity of a graphite more than Lc does. In order to provide good electric conductivity, it is desirable to use graphite including natural and synthetic graphite that have a La greater than 100, more typically between 100 to 300 Angstrom in the cathode of an alkaline cell. Conventional powdery conductive carbons such as acetylene black have small La, typically in the range below 100 Angstrom. These materials, therefore, have low electric conductivity.
After graphitization the crystal sizes such as Lc and La of carbon increase. Physical properties, for example, the electric conductivity can be improved significantly. Some carbon materials, depending on their molecular structure cannot be completely converted to graphite or only partially converted to graphite. Such materials are classified as non-graphitizable or hard carbons, while those carbons that can be easily graphitized are termed graphitizable. The graphitized carbons normally have higher electric conductivity than the non-graphitized hard carbons.
Carbon fibers (not graphites) are synthetic carbon materials taking the forms of fibers or thin strands of carbon material. Such carbon fibers can be classified into four types based on the precursors and processes used during their manufacture: 1) rayon based carbon fibers, 2) polyacrylonitrile (PAN) based carbon fibers, 3) pitch-based carbon fibers (PCF), and 4) catalyzed vapor growth carbon fibers (VGCF). The pitch-based carbon fibers (PCF) can further be classified into isotropic pitch-based and the mesophase pitch-based carbon fiber (MPCF). Most carbon fibers are not graphitizable. However, mesophase pitch-based carbon fiber (MPCF) and vapor growth carbon fiber (VGCF) are graphitizable. The graphitized mesophase pitch-based carbon fibers (GMPCF) generally have a much higher electrical conductivity than MPCF, the non-graphitizable form such as isotropic pitch-based carbon fiber and other carbon fibers such as rayon or polyacrylonitrile (PAN)-based carbon fibers. The diameter of carbon fibers can vary from about less than 1 micron (1xc3x9710xe2x88x926 meter), e.g., in the case of vapor growth carbon fibers to about 5 to 10 micron (5 or 10xc3x9710xe2x88x926 meter) and even up to 100 micron (100xc3x9710xe2x88x926 meter), e.g. in the case of mesophase pitch carbon fibers. Mesophase pitch-based carbon fibers (MPCF) and graphitized mesophase pitch-based carbon fibers (GMPCF), typically have a diameter between about 5 and 10 micron, more typically between about 5 and 7 microns. Conventional mesophase pitch-based carbon fibers and graphitized mesophase pitch-based carbon fibers have a BET surface area between about 0.2 and 5.0 m2/g, more typically 0.5 to 3 m2/g.
Typically, natural and synthetic graphite materials including expanded graphite are in a flaky crystalline form. They can have average particle sizes ranging from about 2 to 50 microns. A suitable flaky natural crystalline graphite having an average particle size of about 12 to 15 microns is commercially available under the tradename xe2x80x9cMP-0702Xxe2x80x9d or xe2x80x9cNdG-15xe2x80x9d from Nacional de Grafite. Suitable expanded graphites typically have average particle sizes ranging from 0.5 to 40 microns. As described hereinabove, expanded graphite can be natural graphite or synthetic graphite wherein the graphite crystal lattice has been uniaxially expanded. Such expansion reduces the crystalline sizes of expanded graphite particles in c-axis direction, but still maintains large crystalline dimension in a-axis direction. The expanded graphite thus can exhibit a much higher aspect ratio (i.e., ratio of thickness to diameter) in term of crystalline size. Meanwhile, expansion in c-direction creates large volume within the graphite particles, resulting in increase in bulk volume and particle size. The large particle size and high aspect ratio of expanded graphite relative to flaky natural or synthetic graphites suggest that expanded graphite can provide an increase in the number of points and/or surfaces that contact each other and as well with the manganese dioxide particles. This in turn can provide enhanced conductivity in cathodes formed from mixtures thereof. Suitable expanded graphites are available commercially, for example, from Chuetsu Graphite under the tradename xe2x80x9cDCN-2xe2x80x9d, Timcal AG under the tradename xe2x80x9cBNB90xe2x80x9d, and Superior Graphite under the tradename xe2x80x9cABG-40. Cathode containing expanded graphite usually has better electric conductivity than that with small particle flaky natural graphite.
The use of expanded graphite as a conductive additive in cathodes of conventional alkaline primary cells is known and disclosed for example, in U.S. Pat. No. 5,482,798. A suitable expanded graphite having an average particle size ranging from 0.5 to 15 microns, preferably from 2 to 6 microns is disclosed in U.S. Pat. No. 5,482,798. The smaller average particle size of expanded graphite relative to conventional natural or synthetic crystalline graphite (e.g., 15 to 30 microns) was hypothesized to facilitate the formation of a conductive network typically at a lower volume fraction of graphite. An expanded graphite having an average particle size greater than about 30 microns was disclosed to provide no performance advantage in alkaline cells compared to a conventional non-expanded natural graphite having a comparable particle size. U.S. Pat. No. 5,482,798 also discloses that suitable amounts of expanded graphite can range from about 2 to 8 weight percent, and preferably from about 3 to 6 weight percent of the total cathode. Further, for expanded graphite contents of greater than about 10 weight percent no performance advantage is provided relative to equivalent amounts of unexpanded graphite particles in alkaline primary cells.
Various methods for preparing mixtures of manganese dioxide and graphite are known to provide suitable levels of electric conductivity in cathodes of alkaline cells. Typically, graphite can be mixed dry or wet with manganese dioxide using any of a variety of conventional blending, mixing or milling equipment. For example, U.S. Pat. No. 5,482,798 discloses the use of a twin cylinder mixer or a rotary tumbling mixer to dry mix graphite and manganese dioxide. In a subsequent step disclosed in U.S. Pat. No. 5,482,798 the formed mixture was wet-pulverized, preferably in water, using a horizontal media mill such as a ball mill or bead mill to decrease average manganese dioxide particle size to less than 10 microns.
Manganese dioxides suitable for use in alkaline cells include both chemically produced manganese dioxide known as xe2x80x9cchemical manganese dioxidexe2x80x9d commonly referenced in the art as xe2x80x9cCMDxe2x80x9d and electrochemically produced manganese dioxide known as xe2x80x9celectrolytic manganese dioxidexe2x80x9d commonly referenced as xe2x80x9cEMDxe2x80x9d. CMD can be produced economically and in high purity, for example, by the methods described by Welsh et al. in U.S. Pat. No. 2,956,860. EMD is manufactured commercially by the direct electrolysis of a bath containing manganese sulfate dissolved in a sulfuric acid solution. Manganese dioxide produced by electrodeposition typically has high purity and high density. Processes for the manufacture of EMD and representative properties thereof are described in xe2x80x9cBatteriesxe2x80x9d, edited by Karl V. Kordesch, Marcel Dekker, Inc., New York, Vol. 1, 1974, pp. 433-488. EMD is the preferred type of manganese dioxide for use in alkaline cells. One consequence of the electrodeposition process is that the EMD typically retains a high level of surface acidity from the sulfuric acid of the electrolysis bath. This acidity in residual surface can be neutralized for example, by treatment with an aqueous base solution. Suitable aqueous bases include: sodium hydroxide, ammonium hydroxide (i.e., aqueous ammonia), calcium hydroxide, magnesium hydroxide, potassium hydroxide, lithium hydroxide, and any combinations thereof. Typically, commercial EMD is neutralized with a strong base such as sodium hydroxide because it is both highly effective and economical.
Thus, even though considerable effort has been expended to improve cathode conductivity, as evidenced by the prior art cited hereinabove, the conductive carbon and/or graphite employed therein require additional refinement in order to improve substantially the discharge performance and service life of alkaline electrochemical cells.
It is a principal objective of the present invention to produce an improved cathode comprising manganese dioxide and graphite material comprising treated graphitized mesophase pitch-based carbon fibers (GMPCF) to provide an improvement in cathode conductivity and discharge performance.
An aspect of the present invention is directed to producing an alkaline primary cell having increased service life as well as improved discharge performance at high rate of discharge, e.g. between about 0.5 and 1-Watt or 0.5 to 2-Amps.
The present invention provides a conductive cathode consisting of predominantly manganese dioxide admixed with a small amount of electrically conductive graphitized mesophase pitch-based carbon fibers (GMPCF). The graphitized mesophase pitch-based carbon fibers are preferably chemically treated before admixing with the MnO2 to produce the treated graphitized mesophase pitch-based carbon fibers (treated GMPCF). Preferably the graphitized mesophase pitch-based carbon fibers are treated with potassium hydroxide (KOH) at high temperature before admixing with MnO2. The treated graphitized mesophase pitch-based carbon fibers when used in admixture with particulate manganese dioxide to form a cathode mixture results in higher conductivity of the cathode mixture for a given amount of carbon. The treated graphitized mesophase pitch-based carbon fibers also function as a binder for the particulate manganese dioxide. Such treated graphitized mesophase pitch-based carbon fibers have a higher BET surface area and are softer and thus less resilient than the untreated graphitized mesophase pitch-based carbon fibers during the compressing process of making the cathode. It is theorized that this combination of factors leads to the higher electric conductivity of cathodes comprising MnO2 and the treated graphitized mesophase pitch-based carbon fibers. Such cathodes are more easily compacted. The softer carbon fibers and higher BET surface area of the fibers appear to result in better intimate contact between the fibers with each other and with the MnO2 particles, in turn leading to higher conductivity.
The treated graphitized mesophase pitch-based carbon fibers (treated GMPCF) of the invention have a mean average BET surface area of between about 10 and 60 m2/g, preferably between about 10 and 50 g/m2. Desirably, the treated graphitized mesophase pitch-based carbon fibers have a mean average BET surface area of between about 30 and 60 m2/g, advantageously between about 30 and 50 m2/g. Such treated graphitized mesophase pitch-based carbon fibers have a length typically of between about 20 and 200 micron, with a mean average length of between about 40 and 150 micron, and a mean average diameter between about 1 and 10 micron, typically between about 4 and 7 micron. The treated graphitized mesophase pitch-based carbon fibers (treated GMPCF) also have a crystal size Lc in the xe2x80x9ccxe2x80x9d direction of between about 50 and 300 Angstrom, for example, between about 50 and 250 Angstrom, more typically between about 100 and 200 Angstrom in the xe2x80x9ccxe2x80x9d direction (crystal thickness), and a crystal crystal size La of between about 100 and 300 Angstrom, typically about 200 Angstrom in the xe2x80x9caxe2x80x9d direction. (The crystal size Lc in the xe2x80x9ccxe2x80x9d direction of the treated mesophase pitch-based carbon fibers of the invention have been determined to be smaller than the crystal size Lc of untreated mesophase pitch-based carbon fibers.)
The weight ratio of graphite material to MnO2 in an alkaline cell cathode, irrespective of the percent by weight of MnO2 in the cathode, is between about 0.05 (1:20) and 0.085 (1:12). The total graphite material in a representative alkaline cell cathode is between about 4 and 10 percent by weight of the cathode. In accordance with a specific aspect of the invention the total graphite material can be composed entirely of the treated graphitized mesophase pitch-based carbon fibers of the invention or entirely of a mixture of treated graphitized mesophase pitch-based carbon fibers and graphite including natural and expanded graphite. Alternatively, the treated graphitized mesophase pitch-based carbon fibers can make up a fraction of the total graphite material, for example, between about 1 and 100 percent by weight, desirably between about 5 and 50 percent by weight of the total graphite material in the cathode. In such case the remainder of the graphite material can be composed of other graphites for example, natural or expanded graphites or mixtures thereof.
The treated graphitized mesophase pitch-based carbon fibers also have the property that makes a cathode mixture comprising MnO2, treated graphitized mesophase pitch-based carbon fibers and aqueous KOH easier to compact. The cathode mixture can be prepared wet by mixing the MnO2, treated and untreated graphitized mesophase pitch-based carbon fibers together with aqueous potassium hydroxide (KOH). The wet mixture can then be compacted and inserted into the cell casing or the wet mixture can be inserted into the cell and compacted while in the cell. Such cathode comprising the graphitized mesophase pitch-based carbon fibers exhibits increased conductivity and high bulk density of the MnO2 (EMD), allowing for a high loading of manganese dioxide (EMD) in the cell.
Alternatively, a mixture of MnO2 and treated graphitized mesophase pitch-based carbon fibers can be mixed while dry and the resulting dry mixture compacted into the cell. Aqueous potassium hydroxide solution can then be added to the compacted dry mixture in the cell. The aqueous KOH is readily absorbed into the dry mixture to form the cathode. This can result in a cathode of increased conductivity and high bulk density resulting in even higher loading of MnO2 therein. Also the compacted dry mixture of MnO2 and treated graphitized mesophase pitch-based carbon fibers can absorb the aqueous potassium hydroxide solution quickly and in a large amount, due to the porous structure of treated graphitized mesophase pitch-based carbon fiber. This can result in a cathode of increased electrolyte absorption and resulting in even higher performance for high rate discharge.
The term xe2x80x9cgraphitexe2x80x9d or xe2x80x9cgraphite materialxe2x80x9d as used herein shall include natural and synthetic graphites, expanded graphites, graphitized carbon, and graphitized carbon fibers. The term xe2x80x9cgraphitized carbon fibersxe2x80x9d shall mean carbon fibers which have a basic graphite crystalline structure as defined by the International Committee for Characterization and Terminology of Carbon (ICCTC, 1982) as published in the Journal Carbon, Vol 20, p. 445. Such graphitized carbon fiber can be obtained by subjecting carbon fiber to a graphitization process, which normally involves heating carbon or carbon fibers at very high temperatures, typically between about 2500xc2x0 C. and 3000xc2x0 C. to obtain the characteristics of the basic graphite structure which is a three-dimensional crystalline structure comprised of ordered layers of hexagonally arranged carbon atoms stacked parallel to each other as referenced in the International Committee for Characterization and Terminology of Carbon (ICCTC, 1982). It will be appreciated that such graphitized carbon fibers can be broadly classified as such despite that their average BET surface area and crystalline size (La and Lc) can be altered by the methods described herein. The term xe2x80x9cnatural crystalline graphitexe2x80x9d shall mean graphite that is minimally processed, i.e., essentially in its geologically occurring natural crystalline form. The term xe2x80x9csynthetic graphitexe2x80x9d as used herein shall mean synthetically prepared or processed graphite. Synthetic graphite can have crystal structure and morphological properties that are the same or similar to natural graphite or can have a different structure. The term xe2x80x9csynthetic graphitexe2x80x9d as used herein unless further qualified is also intended to include various expanded forms of graphite (including expanded graphite that has been exfoliated). The term xe2x80x9cexpanded graphitexe2x80x9d is a recognized term of art, for example, the form of graphite generally as referenced in U.S. Pat. No. 5,482,798. Further, expanded graphite as used herein can be formed from natural and/or synthetic non-expanded graphite processed so as to have a uniaxially expanded crystal lattice. The extent of uniaxial expansion can be sufficiently large such that the expanded graphite particles can completely exfoliate (i.e., separate into thin laminae). The term xe2x80x9cflakyxe2x80x9d as commonly used in connection with graphites, (i.e., natural or synthetic flaky graphites) is intended to reflect that such graphites have a plate-like, non-expanded particle form.
The term xe2x80x9ccarbon fiberxe2x80x9d shall mean elongated strands of carbon having a length to diameter ratio greater than about 4, typically greater than 8 and up to about 30 or more. Mesophase pitch-based carbon fibers is a known and commercially available type of carbon fiber. It is made by thermally treating xe2x80x9cpitchxe2x80x9d. Pitch is a well known material which is a carbonaceous tacky tar residue, typically a petroleum tar residue as defined, for example, in Hawley, Condensed Chemical Dictionary, Tenth Edition. Mesophase pitch is made by chemically treating pitch as described, for example, in U.S. Pat. No. 4,005,183, Japanese Patent 57-42924 (corresponds to U.S. Pat. No. 4,303,631), U.S. Pat. No. 4,208,267, Japanese Patent 58-18421, Japanese Patent 63-120112 (corresponds to U.S. Pat. No. 4,822,587), or in the references Mochida, Carbon, Vol. 13, p.135 1975, Park and Mochida, Carbon, Vol. 27, p.925 (1989), and Mochida, Carbon, Vol. 27, p.843 (1989). The mesophase pitch is an intermediate phase which is a liquid crystal. The mesophase liquid crystal can be formed in tar or pitch by heating such material at elevated temperatures, typically between about 350xc2x0 C. and 450xc2x0 C. The mesophase tar or mesophase pitch resulting from heating such material was first reported and described in Taylor, G. H.,xe2x80x9cDevelopment of Optical Properties of Coke During Carbonizationxe2x80x9d, Fuel, Vol. 40, p. 465 (1961). Mesophase pitch characteristically have molecules which are highly oriented in one direction which makes the material more readily graphitizable. Such molecular orientation of the mesophase pitch is described in Brooks, J. D. and Taylor, G. H., Chemistry and Physics of Carbon, Vol. 4, p. 243 (1968). Thus, the term mesophase pitch shall have the ordinary and accepted meaning as used in the art as applied to pitch which has been heat treated to obtain the mesophase liquid crystalline phase structure with respect to such pitch material. The mesophase pitch can be made into mesophase pitch-based carbon fiber (MPCF) by first thermal extruding the mesophase pitch material at elevated temperature of about 200 to 350xc2x0 C. as, for example, as described in Otani U.S. Pat. No. 4,016,247. The extruded mesophase pitch fiber, by way of nonlimiting example, is then typically subjected to oxidation, preferably by reheating the material in air at 250 to 350xc2x0 C. to oxidize the extruded material as described for example in Otani, Carbon, Vol. 3, p.31 (1965). This material can then be subjected to heat treatment at temperature between 1000 to 1800xc2x0 C., typically 1000xc2x0 C. to 1200xc2x0 C. in the presence of an inert gas such argon as described for example in Otani, Carbon, Vol. 3, p. 31 (1965). The end product is mesophase pitch-based carbon fiber, which is a recognizeable type of carbon fiber known and referenced in the art by such name. Thus, the term xe2x80x9cmesophase pitch-based carbon fiberxe2x80x9d as used in this application is intended to refer to such material as it is ordinarily known and referenced in the art.
The mesophase pitch-based carbon fiber can then be graphitized by conventional methods used to graphitize carbon. The graphitization process normally involves heating the fiber at very high temperatures typically between about 2500xc2x0 C. and 3000xc2x0 C. as is known in the art. Such graphitization processes, by way of non limiting example, can involve treating the carbon fibers with heat to a temperature of above 2500xc2x0 C., more desirably between 2800xc2x0 C. and 3000xc2x0 C., or at temperature sufficient to obtain the characteristics of an ordered three-dimensional graphite crystalline structure consisting of layers of hexagonally arranged carbon atoms stacked parallel to each other as defined by the International Committee for Characterization and Terminology of Carbon (ICCTC, 1982), published in the Journal Carbon, Vol. 20, p. 445. The graphite is further characterized by ordered d-spacing between graphite layers (ordered layers), and crystalline sizes Lc and La in the c and a directions of the crystalline structure, respectively. Thus, the term graphitized mesophase pitch-based carbon fibers (GMPCF) is a type of graphite material and shall mean mesophase pitch-based carbon fibers that have been subjected to such conventional graphitization processes.
The term xe2x80x9ctreated graphitized mesophase pitch-based carbon fibersxe2x80x9d (treated GMPCF) shall mean graphitized mesophase pitch-based carbon fibers that have been subsequently treated to increase the BET surface area thereof. Such treatment, by way of non-limiting example, shall include heating the graphitized mesophase pitch-based carbon fibers with potassium hydroxide, KOH, sodium hydroxide, NaOH, or other oxidant agents at elevated temperatures. The treatment method includes methods that were disclosed in the reference of U.S. Pat. No. 4,082,694 (Wennerberg) and U.S. Pat. No. 4,039,473 (Schafer) which are described therein as applied to other types of carbons and not to graphitized carbons. Specifically, there is no contemplation in these references for treating a graphitized carbon fiber with KOH at high temperature to produce an improved graphitized fiber for application into the cathode of an alkaline cell to improve the electrical conductivity of the cathode. U.S. Pat. No. 4,946,663 (Audley) discloses treatment of carbonized fibers or carbonized carbon mats such as xe2x80x9ccharcoal-clothxe2x80x9d or simply non-carbonized material carbon fiber precursor such as rayon with KOH at high temperature in order to increase the surface area of such material. The fibers treated with KOH in either case are not graphitized. There is no mention or concern in this reference with treating graphitized carbon fibers with KOH. There is no mention or contemplation in this reference or the references U.S. Pat. No. 4,082,694 (Wennerberg) and U.S. Pat. No. 4,039,473 (Schafer) of treating a graphitized carbon fiber with KOH at high temperature to increase both BET surface area of such graphitized carbon fiber and increase the electrical conductivity of a cathode employing such material.
In the present invention it has been determined that such treatment methods when applied to graphitized mesophase pitch-based carbon increases the BET surface area of the graphitized mesophase pitch-based carbon fibers and increases the electrical conductivity of cathode mixtures comprising MnO2 (EMD) and graphitized mesophase pitch-based carbon fibers.
The term BET surface area (m2/g) as used herein shall mean the standard measurement of particulate surface area by gas (nitrogen and/or other gasses) porosimetry as is recognized in the art. The BET surface area measures the total surface area on the exterior surface of the particle or fiber and also that portion of surface area defined by the open pores within the particle or fiber available for gas adsorption and deadsorption when applied. BET surface area determinations (Brunauer, Emmett and Taylor method) as reported herein is carried out in accordance with ASTM Standard Test Method D4820-99. The graphite powder or graphitized carbon fibers can be outgassed under vacuum at 150xc2x0 C. for 7 hours in an instrument such as Quantachrome Autosorber Model 6 manufactured by Quantachrome Co. The BET surface area can be determined from nitrogen gas adsorbate and use of a multi-point BET equation to calculate the BET surface using the software provided by the instrument manufacturer.